1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image transferred to a transfer material and an image forming apparatus employing such a fixing device.
2. Description of the Related Art
A fixing device as disclosed in Japanese Unexamined Patent Publication No. 2002-82571 has been conventionally known. This fixing device is arranged at a side immediately downstream of an image forming assembly in an image forming apparatus and fixes a toner image transferred to a sheet (transfer material) in the image forming assembly to this sheet by heating, i.e. applies a so-called image fixing operation. Such a fixing device includes a heating roller provided with a heat source inside, a fixing roller opposed to this heating roller and a fixing belt mounted between the fixing roller and the heating roller. There is also provided a pressure roller to be pressed into contact with the fixing roller with the fixing belt therebetween.
The sheet having the toner image transferred thereto is fed to a nip portion formed between the pressure roller rotating about its central axis and the fixing belt turning between the heating roller and the fixing roller. Upon passing the nip portion, toner particles are melted by supplying heat from the fixing belt to the sheet, and the image fixing operation is performed by fixing the melted toner particles to the surface of the sheet.
In such a fixing device, the temperature of the fixing belt needs to be properly controlled. To this end, a temperature sensor is arranged at a position facing part of the outer circumferential surface of the heating roller where the fixing belt is not mounted, and the temperature of the fixing belt is controlled to a specified temperature based on the detection result of this temperature sensor.
In the operation of fixing the toner particles to the sheet, temperature at the outer surface of the fixing belt to be directly brought into contact with the toner particles influence the quality (fixing property) of the image fixing operation. However, immediately after the warm-up following the start of the image forming apparatus, the outer surface of the fixing belt has not yet reached the specified temperature in many cases. If the sheet is passed through the nip portion in such a state, the temperature at the outer surface of the fixing belt falls to or below a specified temperature, thereby causing inconvenience of deteriorating the fixing property.
Even if sufficient time passes after the warm-up and the fixing belt is uniformly heated up to the specified temperature, the fixing property is deteriorated in some cases. For example, if a large number of sheets are successively fed to the nip portion per unit time, the outer surface of the fixing belt is deprived of a large quantity of heat. A resulting fall in the surface temperature of the fixing belt may deteriorate the fixing property of the toner particles.
In order to avoid such inconvenience, the temperature sensor (temperature detecting means) is arranged at the position facing the part of the outer circumferential surface of the heating roller where the fixing belt is not mounted in the fixing device of the above patent publication, and the temperature of the fixing belt is controlled to the specified temperature based on the detection result of this temperature sensor. However, the temperature on the outer circumferential surface of the heating roller does not necessarily correspond to the temperature at the outer surface of the fixing belt upon the occurrence of the above inconvenience. This leads to a problem that the surface temperature of the fixing belt cannot be properly controlled and the fixing property of the toner particles is deteriorated in some cases.
In the fixing device of the above patent publication, the heating roller might be overheated if the temperature sensor cannot precisely detect temperature for a certain reason or a control circuit is in trouble.